See if this makes sense (and no finger pointing and guffawing if it doesn't 'cause I never claimed to be resident expert, so there)

Can't the whole construction 'mother-in-law' be a noun? The term names one particular person. Without the 'in-law' you are talking about a completely different entity.

I see it the same way I see the term highrise. High is a descriptive but having been joined to rise becomes part of a noun. True the hyphen has been removed but isn't that just a matter of time.

Thus Mother-in-law's house would fit the bill.

Oh wait, I just thougth of the plural. We'd say Mothers-in-law wouldn't we? Ah heck. Colour me clueless again.

I'll stick to "belle-mère / belles-mères". This is the first time I can say it is so much clearer in French.